User:H. Sebastian Seung/Proposed/Persistent activity

Neural activity related to short-term memory has been recorded in a wide variety of
brain areas, chiefly in awake behaving primates. (citations needed here)
Typically this activity is evoked by a
transient sensory stimulus, and persists as an elevated or suppressed discharge for up
to many seconds until a behavioral response (figure needed here). As many studies of
neural coding have shown, this persistent neural activity is correlated with
information stored in short-term memory during the time interval separating stimulus
from response.

Both cellular and circuit mechanisms for persistent neural activity have been
hypothesized. The most popular candidate for a cellular mechanism is bistability due to plateau potentials or synaptic
properties. Since plateau potentials last for seconds, and can be turned on or off rapidly by a transient input, they seem
ideal for supporting persistent neural activity. The most popular candidate for a circuit mechanism has been reverberating
activity in synaptic feedback loops, as hypothesized by Hebb and Lorente de No. This hypothesis has been
implemented in a class of models known as attractor neural networks, which have multiple stable states corresponding to
persistent activity patterns.

Both of these hypotheses (multistable cells and multistable circuits) are well-known and appealing in their simplicity.
Some neuroscientists casually accept one or the other as true. It is less widely known that both hypotheses, at least in their
simplest formulations, are problematic. Both have trouble accounting for graded forms of persistent activity
that have been observed in certain brain areas. Transient inputs can flip a bistable cell or a bistable circuit back and forth
between two states, but this does not resemble the examples of persistent activity that are continuously graded.